The present invention relates to a prosthetic intervertebral disc nucleus and to an insertion device for use therewith. Such devices are useful to replace damaged disc nuclei, whether in the lumbar or other region of the spine.
The normal intervertebral disc is a highly specialized joint between the intervertebral bodies from the second cervical vertebra to the first sacral vertebra in the human being. A disc is made up of a strong outer ring called the annulus which is strongly attached to the intervertebral bodies above and below through collagen fibers, and a central nucleus. The nucleus comprises a mesh of collagen fibers to which is attached proteoglycan molecules which are hygroscopic. It can therefore be said to consist of a central portion and an intermediate zone. The central portion comprises 90% proteoglycan and 10% collagen, and the intermediate zone rather less proteoglycan and proportionately more collagen. The annulus comprises 90% collagen with about 10% proteoglycan, which proteoglycan acts to allow a sliding motion between adjacent layers during normal daily use.
Like all other joints in the body, intervertebral discs, particularly lumbar intervertebral discs, are subject to various types of injury, degeneration and disease. Painful disc syndromes can develop due to the destruction of the intervertebral disc structure.
It is often the case that back pain emanating from an intervertebral disc arises from a damaged annulus because the annulus itself is the only part of the disc structure which is innervated. In theory then, the insertion of a prosthesis which has itself a finite structure with inherent strength but at the same time is held in position by the annulus would permit the annulus itself to heal with the commensurate relief of pain.
Various disc prostheses are known in the art for example from U.S. Pat. No. 3,875,595 and WO 95/31948. There are various problems associated with these. In U.S. Pat. No. 3,875,595, there is provided a collapsible plastic bladder prosthesis of the same exterior form as the nucleus pulposis of an intervertebral disc. This is provided with a stem through which liquid/plastic is introduced to inflate the prosthesis to a natural form. The difficulty with this arrangement is that since the exterior of the bladder-like prosthesis is impermeable, the prosthesis is not anchored and hence must be provided with external studs to secure the same in the position relative to adjacent vertebrae. Such an arrangement tends to adversely impact upon the adjacent vertebrae and/or the plastic sheath of the prosthesis, and bearing in mind the trauma associated with insertion and subsequent repair where necessary, such arrangements have not been found to be satisfactory.
In part, these problems have also been addressed in WO 95/31948. This provides an expandable fabric insert for stabilizing spinal motion which allows through growth of living cells, unlike other prior art prostheses. This can be inserted by open operation. This arrangement is designed to be filled with bone graft or bone substitute material with a view to bringing about a solid bone fusion or a fibrous fusion. For anchoring purposes it is of course necessary for the living cells to grow through the woven material of the prosthetic cover and this in fact can occur.
Alternatives have been suggested, for example in U.S. Pat. No. 5,674,295. This describes a nuclear prosthesis which has an outer restraining jacket surrounding a hydrogel, and optionally other materials. In the dehydrated state, the hydrogel has a volume smaller than the constraining jacket but when hydrated is constrained within the jacket so it can conform to a certain extent to external loads placed upon it. The prosthesis is always of a smaller volume than the nuclear space in which it is placed, but on hydration increases the disc height, tensioning the fibers of the annulus.
Another attempt at this has been in WO 92/10982 in that a prosthesis of the type of this invention is revealed with a supported membrane being approximately above 15,000 daltons, or 25A. While this approach may be viable, better results, especially in terms of immunological disturbances may be achieved if significantly lower porosity is utilized without rendering the cover completely impermeable.
What is required therefore is a prosthesis which can be introduced with the minimum of trauma via the lateral percutaneous route, which stays correctly anchored in position, which allows some exchange of fluid and low molecular weight materials, but without immunological problems, and/or acts over the majority of the articulating surfaces of the adjacent vertebrae.
The present invention is directed towards an intervertebral disc nucleus prosthesis generally comprising a transudative or ion transport material extended over the inner face of a prosthetic woven or porous fabric chosen for its strength and immunological neutrality. With this design, tissue can grow over and incorporate the outer fabric, while water and very low molecular weight materials can pass the prosthetic fabric and the transudative membrane, thus eliminating, or at least substantially alleviating, any immunological problems. This allows the anchoring of the replacement nucleus pulposis within a disc space while preventing the ingrowth of bony trabeculae, thereby providing a more satisfactory long term solution.
An alternative embodiment generally comprises a three layer cover, wherein the outer and inner layer are formed of a strong material in a sandwich construction with a middle layer of a transudative material of a small pore size. Very high pressures are present in use within the normal disc nucleus. To counteract this it is often necessary to introduce a hydrogel or other suitable substance into the cover in a liquid or powdered state. Thus, the hydrogel would have a tendency to escape through the cover were it not so constructed as to combine strength with an ability to retain the hydrogel which might otherwise leak out through the pores in the cover. The alternative jacket construction addresses this concern.
According to a first aspect of the invention therefore there is provided a prosthetic cover shaped to form a replacement nucleus pulposis of an intervertebral disc. The cover comprises a permeable layer of an immunologically neutral material and a transudative material adapted to allow the through flow of selected low molecular weight materials only. The cover terminates in a valve structure configured to allow the normally irreversible introduction of a hydrogel material. In one preferred embodiment, the valve structure includes a one way valve. In another preferred embodiment, the prosthetic cover comprises a three layered cover, preferably forming a sandwich construction having a middle layer of a transudative material.
The term xe2x80x9cnormally irreversiblexe2x80x9d it will be understood refers to the situation in use. Clinical situations can arise wherein it is desirable to change the volume of hydrogel in the cover post-operatively. This may be achieved by causing the hydrogel to flow out of the cover by re-opening the valve structure in a fashion analogous to the filling operation, liquidizing the hydrogel and then applying a vacuum.
Regardless of the number of layers comprising the cover, the cover further includes in one preferred embodiment, a strengthening member opposite the valve structure. The strengthening member is preferably integrally formed to the material comprising the cover and is configured to selectively receive an end of an introducer rod during implantation of the prosthesis. In this regard, the strengthening member allows a user to apply a pushing force on the cover without causing any tears.
As is clear from the foregoing, the low molecular weight materials may include water and other low molecular weight materials present in the environment of use. The immunologically neutral material may be woven and is particularly satisfactory if selected from xe2x80x9cDacronxe2x80x9d(copyright) and xe2x80x9cGortexxe2x80x9d(copyright), but other similar materials with the same MW cut off characteristics are also suitable, for example those described in U.S. Pat. No. 5,674,295.
The transudative material membrane is preferably adapted to have a molecular cut off below 100 Angstroms, or 12,000 daltons, more preferably below 9,000 daltons. In particularly preferred embodiments, the transudative membrane material has a molecular cut off of up to MW500, but may be as low as MW200 or even MW50. In a particularly favoured composition the transudative membrane material may comprise xe2x80x9cOpsitexe2x80x9d(copyright).
The valve structure is preferably formed of an imaging transparent material, for example titanium, carbon fibre or a durable biocompatible plastics material such as polypropylene. In one preferred embodiment, the valve structure includes a one way valve arrangement which may be a flap valve partially attached to an inside of the cover, making it more certain that the valve structure will be in a closed state when an internal pressure of the cover exceeds an exterior or injection pressure of the hydrogel material. Alternatively, the one way valve arrangement may be a conical nose with a narrower internal opening, again directing the valve structure to a closed state under similar circumstances. The conical configuration facilities insertion of an introducer rod, as described below.
In one alternative embodiment, the valve structure further includes an extension body attached to the one way valve arrangement. The extension body may be external to the cover, or partially or totally inside it. With one preferred option, the extension body is external or partially external relative to the cover to allow attachment of an external introducer tube that controls the whole process of implantation, as described below. The extension body preferably has a central longitudinal bore which is provided over a part thereof with an internal screw thread.
The hydrogel is preferably a polyvinyl alcohol material, such as xe2x80x9cHYPANxe2x80x9d(copyright), developed into a fluid or liquid form which will easily pass through the valve structure and subsequently harden. Ideally, the swelling pressure of the resultant hydrogel is in a range similar to, or as close as possible to, a normal lumbar intervertebral disc.
The prosthesis of the present invention is preferably sized such that an internal surface area of a nuclear cavity is virtually the same as the prosthetic cover. This will ensure that load distribution within the resulting prosthesis is similar to that of a normal intervertebral disc. The technology of hydrogels at the present time means that the swelling pressure of hydrogel can only approximate to one quarter or one third of a normal disc. So to retain their ability to maintain disc height, it is preferable to form the hydrogel as a solid material, or that the hydrogel harden or xe2x80x9ccurexe2x80x9d following injection into the cover. In one alternative embodiment, a fine wire of a radiolucent material is incorporated within the prosthesis to demonstrate the position of a prosthesis in vivo.
In use, one preferred method of insertion of a prosthesis in accordance with the present invention is as follows:
A skin incision is made adjacent a damaged intervertebral disc, including an annulus and a nucleus, which has previously been extensively imaged by a Computer Tomography or Magnetic Resonance Imaging. If necessary, confirmation that it is the disc which is painful may be reached by effecting provocative stress discography. This allows percutaneous disc surgery to be carried out by a lateral approach whereby a cannula or trochar is used to insert instruments laterally between adjacent vertebrae in the spine through the paraspinal musculature so entering the disc at the postlateral corner in the xe2x80x9csafexe2x80x9d triangle; inferior to the exiting nerve root. The incision provides for access to the nucleus portion of the intervertebral disc.
Chymopapain may be injected into the nucleus to digest the proteoglycan structure thereof. Mechanical action as by a brush with polypropylene bristles may be used to aid the breakdown of any remaining collagen structure to enhance the effect of chymopapain which may then be removed by suction. Subsequently an intervertebral disc nucleus prosthesis in accordance with the present invention is introduced through the disc annulus. The disc annulus comprises an outer ring of strong collagenous fibrous tissue. As previously described, the prosthesis preferably includes a cover and a valve structure. The valve structure, in one preferred embodiment, includes the one-way valve arrangement (or conical nose) and an extension body that may be knurled or fluted as appropriate. A strengthening member may be incorporated into the cover immediately opposite the valve structure for receiving an introducer rod. This allows the relatively atraumatic insertion of the prosthesis cover through the annulus into the space created by the removed nucleus.
Prior to insertion, the prosthesis is preferably attached to an external introducer tube. In one preferred embodiment, a distal end of the external introducer tube is internally threaded to selectively engage external threads of the extension body. Thus, the external introducer can be selectively secured to the valve structure. Additionally, a tubular screw driver may be provided. The tubular screw driver is sized to be coaxially received within the external introducer tube. Further, a distal end of the tubular screw driver is preferably configured to selectively mate with both the extension body of the valve structure, as well as with the distal end of the external introducer tube. With this preferred design, the tubular screw driver controls actuation of the valve structure and attachment between the valve structure and the external introducer tube. Finally, the introducer rod is coaxially positioned within the external introducer tube.
In one preferred embodiment, the introducer rod is preferably externally threaded to threadably engage an internal thread on the longitudinal bore of the valve structure. The introducer rod serves as a temporary stiffening device, allowing a surgeon to apply a pushing force on the cover. Thus, with proper positioning of the introducer rod, which may be seated in the strengthening member of the cover, the surgeon can extend the prosthesis cover into the cavity between adjacent vertebrae.
Once the prosthesis is positioned within the disc space, the introducer rod is withdrawn. Hydrogel material is then introduced into the prosthesis cover via a syringe connected to the external introducer tube. In one preferred embodiment, a distal end of the syringe is directed through the external introducer tube and secured to the valve structure. With this approach, the syringe has an internal seal, to ensure that the hydrogel material passes through the valve structure into the cover, and a locking mechanism to ensure a tight seal with the valve structure. The syringe of this embodiment further includes a tubular piston rod and a piston that is selectively secured to a screw configured to immediately close the valve structure after injection of the hydrogel. In one embodiment, once a desired volume of hydrogel has been injected into the cover of the prosthesis, and the piston is at the bottom of the syringe, a screw driver may be passed down a center of the piston to insert and tighten the screw to the valve structure. Alternatively, the screw and the screw driver may be incorporated together into the piston, and the piston rod simply turned to secure the screw to the valve structure.
Alternatively, the external introducer tube may be a cannula including two proximal ports to facilitate injection of the hydrogel material. With this configuration, a syringe is secured to one of the two proximal ports. Hydrogel material is forced from the syringe into the external introducer tube. The external introducer tube, in turn, directs the hydrogel material to the valve structure and then into the prosthetic cover. With the prosthesis filled adequately with hydrogel material to a desired internal pressure, a screw is then passed through the external introducer tube and secured to the valve structure so as to retain the contents of the prosthesis. In the event that the volume of hydrogel material needs to be subsequently altered, this can be performed in a substantially non-traumatic way by merely removing the screw and replacing the contents of the prosthesis cover as necessary.
Another aspect of the present invention relates to a valve structure adapted for use in a prosthetic device. The valve structure preferably comprises a valve body with a longitudinal bore therein, obturating means associated with said bore and attachment means. The valve body is configured to be fluidly secured to a cover of the prosthetic device. For example, an exterior portion of the valve body may be attached to the cover such that the longitudinal bore is in fluid communication with an interior of the cover. The obturating means is configured to selectively allow passage of filler material, such as hydrogel through the longitudinal bore. Finally, the attachment means is configured to selectively engage a material injection tool. The material injection tool is preferably an external introducer tube, cannula or similar apparatus designed to selectively engage a distal end of the material injection tube. In one preferred embodiment, the material injection tube is adapted to accommodate an introducer rod while also allowing injection of material to the valve structure.
The longitudinal bore of the valve body is preferably internally screw threaded for engagement with either the obturating means or a introducer rod, a syringe, material injection tube or screw as appropriate. The valve body may be generally symmetrical and the bore may extend axially within the valve body. Preferably the valve body is formed of an imaging transparent material, for example titanium, carbon fibre or a durable, biocompatible, plastics material such as polypropylene.